[4.5] Handle summer time jumps in event recurrences

[phil-davis]

Backport #653 to 4.5 release branch

ToDo:

• remove PHP 7.4 features used (4.5.x releases support all the way back to PHP 7.1)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.65%. Comparing base (eb2c41e) to head (b7f45de).
Report is 4 commits behind head on 4.5.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.5     #660      +/-   ##
============================================
+ Coverage     98.38%   98.65%   +0.27%     
- Complexity     1813     1907      +94     
============================================
  Files            71       71              
  Lines          4334     5294     +960     
============================================
+ Hits           4264     5223     +959     
- Misses           70       71       +1     

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[phil-davis]

Note: the original PR had 19 commits. This only has 18 commits because one of the 19 was just a single-character format change, which I ended up including with the commit before it when cherry-picking and resolving conflicts etc.

The "missing" commit is:
b37ef3d

[phil-davis]

And the new 19th commit is to remove PHP language features that are not available in PHP 7.1 7.2 7.3

[phil-davis]

Moving forward...